There is a need for solid state sensors for use in many applications, including medicine, biotechnology, biomedical diagnosis, medical therapy and chemical analysis, as well as environmental and industrial monitoring. There is also a need for sensors that are small, light weight and which have a fast response. By integrating sensors with signal processing components in an integrated circuit (IC), very small sensors may be made. CMOS fabricating techniques have the advantage of offering potentially low cost devices that can be mass-produced. CMOS fabricating techniques also offer the potential for devices that require relatively less power to operate, are small, light weight, and fast. The present invention may use these techniques to provide a pH-change sensor.
The ion sensitive field-effect transistor (ISFET), as we know it today, was invented by Bergveld (Bergveld, IEEE Tran. Biomed. Eng., vol. 17, (1970), pp. 70-71), as a solid-state silicon-based chemical sensor. Since then, there has been significant research and development in extending the principle by integrating active recognition materials with ISFETs to form biologically-modified ISFETs (BioFETs), enzyme-modified ISFETs (EnFETs), immunologically-modified ISFETs (ImmunoFETs), DNA-Modified or gene-modified ISFETs (GenFETs) or chemically-modified ISFETs (ChemFETs). Each of the above-mentioned devices are selective to a specific analyte depending on the recognition material present on the ISFET. Most prior implementations of ISFETs with integrated recognition elements are discrete devices with specialized processing requirements that limit commercialization of the technology. In addition, the recognition materials or elements used in such devices may be suitable only for a particular analyte, thereby limiting the diversification of the sensors based on the same sensor platform. A detailed review of recent advances in such devices can be found in Schoning and Poghossian, Analyst, vol. 127, (2002), pp. 1137-1151.
A number of research groups have previously studied different methods of fabricating ISFETs using a modified standard CMOS process with additional process steps or using a standard CMOS process. Bousse et al., IEEE Electron Devices Letters, vol. 9, (1988), pp. 44-46, first proposed the combined fabrication of ion sensors and CMOS circuits on the same IC and more recently, Canè, et al., Sensors and Actuators, B, vol. 35-36, (1996) pp. 136-140 and Bausells, et al., Sensors and Actuators B, vol. 57, (1999), pp. 52-62, have used a multilayer gate structure for the ISFET for fabrication in a commercial CMOS processing technology.
In order to realize a pH-change sensor, an important component is the reference electrode, which provides the electrical contact to the test electrolyte and that provides a proper electrode-electrolyte potential irrespective of the composition of the electrolyte. In most of the previous implementations, the reference electrode was an Ag/AgCl electrode, and due to its size and complexity this electrode severely limited the applicability and the advantages of developing ISFETs in a CMOS process.
A practical solution to the above problem is to use a differential ISFET configuration with a noble metal electrode (either gold or platinum) as the pseudo or quasi reference electrode. Several implementations of this differential ISFET configuration include either using an ISFET and a reference field-effect transistor (REFET) (e.g. Errachid, et al., Sensors and Actuators B, vol. 60. (1999), pp. 43-48, Bergveld, et al., Sensors and Actuators B, vol. 18, (1989), pp. 309-327), or using two ISFETs with different pH sensitive layers (e.g. Wong and White, IEEE Trans. Electron Devices, vol. 36, (1989), pp. 479-487). However, both of these methods need post-processing of the commercially fabricated CMOS integrated circuit because a special pH insensitive layer must be deposited over the ISFET, or a different pH sensitive layer must be deposited on one of the ISFETs. This additional processing is specialized, and therefore negatively impacts the low cost and mass-producing capabilities of using a commercial CMOS process.